Etude des cinétiques de dégradation des résidus agro-alimentaires par la flore bactérienne ruminale, in vitro, en présence de différentes sources d’azote.

Abstract

Five fermentation media supplemented with different nitrogen sources have been used in this work in order to study their effect on the fermentation of two by products from the agri-food industry and two fibrous plants. The first medium is a simplified medium that contains ammonium bicarbonate (source of inorganic nitrogen); the second medium contains sodium glutamate (source of organic nitrogen); the third medium contains potassium nitrate (source of inorganic nitrogen), the fourth medium contains yeast extract (source of organic nitrogen) and the fifth medium is a control without nitrogen. In addition to sodium bicarbonates as the main buffer in all media. Our objective is to measure the in vitro fermentability of four substrates: orange residues, tomato residues, milk thistle (Silybum marianum) and chrysanthemum (Chrysanthemum coronarium) by the bacterial microbiota of dairy cows. In a first experiment, we evaluated the chemical composition of the four substrates such as: dry matter, organic matter, mineral mater, crude protein, plant wall components, total sugars. As well as the fermentation profiles during 120 h long. While in a second experiment, we analyzed the degradability of dry matter, the degradability of neutral detergent fiber fraction, the production of ammonia nitrogen and the production of volatile fatty acids after a 24-hour incubation of orange residues and milk thistle in three different media from the nitrogen source, the first one is used as a control N1 (without nitrogen), the second medium contains ammonium bicarbonate,N2 and the third medium contains sodium glutamate, N3. Thus the chemical composition shows that tomato residues contain the highest crude protein content (9,03 ± 0,9 % DM) and the lowest amount was recorded in chrysanthemum (2,43 ± 0,15 % DM). In addition, the plants contain higher values in NDF compared to by products, we observed 78,98 ± 0,33 % DM in chrysanthemum and 68,8 ± 0,78 % DM in milk thistle while in orange residues it was 15,7 ± 0,09 % DM and tomato residues 33,09 ± 0,35 % DM. The orange residues and tomato residues have more total sugars (40,07 ± 4,69 and 28,49 ± 5,5 % DM, respectively) than fibrous plants (8,42 ± 1,53 and 12,73 ± 4,75 % DM in milk thistle and chrysanthemum respectively). After 120 h of fermentation, the final pH of mixtures (substrate + incubation medium) is in the optimal range 6,38-7,28. Tomato residues and milk thistle have the significantly (p < 0,05) highest fermentability in the media containing an organic nitrogen source: sodium glutamate and yeast extract. The degradability of orange residues and tomato residues is higher than the degradability of milk thistle and chrysanthemum in all media because of their chemical composition where there is a high content of non fibrous carbohydrates and low fiber content, which allowed rumen bacteria to easily and rapidly degrade by products. However, we notice that orange residues are statistically (p < 0,05) most favorable for degradation in all media compared to other substrates. In vitro fermentation analysis of substrates is similar with both statistical models, but it is advisable to use the logistic model which is closer to the experimental results. According to the results of the second experiment, orange residues and milk thistle are favorable for ruminants and will not cause any risk of acidosis despite the high production of volatile fatty acids in N3 media (sodium glutamate) relative to the control N1.